Platinum is known to be a highly active catalyst of many low-temperature electrochemical processes. Notwithstanding its high price, platinum can be excluded from the composition of electrodes in most cases, especially if they are used in acidic conditions (B. B. Damaskin, O. A. Petriy. Electrochemistry. Moscow: Higher School, 1987, 151 p., N. V. Korovin, Fuel cells and electrochemical power units. Moscow: MEI Publishing House, 2005. 280 p.). For instance, this refers to the technology of fuel cells and electrolysis cells for water decomposition with a hard polymeric electrolyte.
Most known methods of catalyst production for fuel cells with a hard polymeric electrolyte with nano-particles of platinum alloys on a carbonic carrier are chemical, e.g., through reduction of platinum salts and transition elements on a carbon carrier from a solution (RF patent No. 2367520, NanoKhim LLC, published on 20 Sep. 2009, IPC B01J23/42). There are electrochemical methods of platinum application onto the carbon carrier (RF patent No. 2455070, N. V. Smirnova et al., published on 10 Jul. 2012, IPC B01J37/34). There are also physical methods to form catalysts of electrodes of electrochemical devices based on platinum on the carbon carrier (RF patent No. 2421849, Institution of the Russian Academy of Sciences Physics and Technology Institute named after A. F. Yoffe, published on 20 Jun. 2011, IPC H01M4/88).
In all the above examples, the catalyst obtained on a carrier (soot) is applied using an air brush on an inert substrate to obtain a fuel cell electrode. Disadvantages of these methods include the use of toxic organic solvents, complexity and high number of production stages, irrational use and platinum carry-over when sputtering, which finally prevents from reaching high specific catalytic activity.
In order to avoid the above drawback, recently magnetron sputtering in vacuum has been applied to obtain a catalyst with low platinum concentration. It has been found that this technology known for a long time allows producing a coat by exclusive bonding on various surfaces with high continuity, irregularity and repeatability of the composition of the deposited layer. The conditions prevalent in magnetron sputtering have a special impact on electrochemical properties of manufactured electrodes.
A similar approach has been used in RF patent No. 2428516, FGUP CNII KN Prometey, published on 10 Sep. 2011, IPC C23C14/35, in order to produce electrodes for electrochemical treatment of water media including catalyst sputtering on a substrate made of inert material, such as titanium, in a vacuum chamber in a gas medium containing oxygen. According to the invention, the metallic substrate is pre-heated in vacuum to 400-450° C., then the metallic composition of the system (Ti—Ru), (Ti—Ru—Ir), (Zr—Ru) is spattered by magnetron method in a medium containing flame-forming argon gas and oxygen reaction gas, argon pressure being maintained constant during the entire sputtering process, and partial oxygen pressure being varied according to the linear law from 0 to 8×10−2 Pa for 10 minutes, and when the oxygen pressure stabilizes, the described catalytic composition is sputtered to achieve the required coating thickness. The magnetron current density exceeds 0.20 A/cm2. This method allows producing a coating showing high adhesion of the catalytic coat to the carrier. A disadvantage of this method for producing electrodes of fuel cells and electrolytic cells is that the catalytic coating produced is dense and has no developed electrochemical surface. This occurs because catalyst particles on the heated substrate have high energy and are rapidly grouped in dense layers during catalyst formation. Using such catalytic layers in fuel cells and catalytic cells with hard polymeric electrolyte is inefficient, since it prevents from reaching high power densities (for a fuel cell) and low energy consumption (for water electrolysis). It is intended to use as anodes in devices for electrochemical treatment of water media and cathode protection.
According to US Patent Application No. 20130052370, DREUX AGGGLOMIRATION, published on 28 Feb. 2013, IPC H01M4/88, there is a method of applying a catalytic layer on a substrate to produce electrodes of fuel cells to ensure improved catalytic efficiency. The catalytic layer is obtained by sputtering of ionized plasma in vacuum when the catalyst in ionized form is applied on the substrate. For this purpose, the substrate is placed in a vacuum chamber where a cathode supporting the target containing the catalyst is also placed. Plasma is preferably generated from argon, supposedly with addition of low amount of hydrogen, nitrogen, oxygen and/or inert gases. The argon pressures is 10−3 to 1 Mbar (which corresponds to 0.1-100 Pa). The plasma is generated by magnetron. For sputtering, all possible combinations of clean targets and/or alloys can be used either alternately (at least from two targets) or simultaneously (at least from two targets). As a catalyst, platinum, palladium, platinum alloys, non-platinum metals, alloys thereof, nitrides or oxides thereof can be used.
This application contains no evidential information concerning the parameters and methods of catalyst production (platinum evaporation parameters in vacuum, density of UHF exposure, etc.). Only the frequency of 13.56 MHz is specified, which is known in the prior art and used in magnetron sprayers, along with 2.4 GHz used in household microwave ovens (http://electrik.info/main/fakty/666-mnogolikaya-mikrovolinovka-gotovit-pischae-izluchaet-mify.html http://avacuum.ru/rus/components/magnetrons/).
There is also no justification to the facts of producing a catalyst with high activity. Only the maximum specific power of the fuel cell with a hard polymeric electrolyte (PEM FC) of 0.855 W/cm2 and platinum specific charging of 0.038 mg of Pt/cm2 are given. No conditions of membrane-electrode blocks are given (temperature, pressure, composition and amount of catalyst on anode and cathode, membrane and gas-diffusion layer parameters, amount of the ion meter in the catalytic layer), and no conditions to obtain such high values are given (temperature, pressure and humidity of gases, voltage-current characteristic). According to the above, it should be noted that it is impossible to obtain data on specific activity of catalytic layers obtained by the method known in the art.
According to U.S. Pat. No. 3,773,639, published on 20 Nov. 1973, IPC C23C15.00 (prototype), there is a method of applying precious metals or their oxides on a metallic substrate by cathodic sputtering in such a way that the substrate's electrochemical activity is increased. The metallic substrate is bombarded with ions in reduced pressure residual atmosphere of pure rare gas (at the partial pressure of 5.33-6.67 Pa), and until high temperature falls down, a precious metal or its oxide is applied on the substrate by cathode sputtering. The sputtering includes two stages at 3000 V and 1.8-2.0 W/cm2. The first stage is performed at 300-500° C. for 30 seconds to 5 minutes in a reduced pressure residual atmosphere of pure rare gas, and the second stage is done for 2-30 minutes in the atmosphere of rare gas and oxygen at the oxygen partial pressure from 0.1 to 25%. Introducing oxygen at this stage allows applying a precious metal (or its oxide) in microcrystalline or porous form with a large specific surface area. Tantalum, zirconium, niobium, titanium or their alloys are used as a substrate. As a precious metal for sputtering, platinum, iridium, palladium, ruthenium, osmium, rhodium and their alloys are selected, individually or in combination.
Along with that, power ranges at the second stage of sputtering in the oxygen atmosphere in the patent are too high and prevent from producing a nano-dispersive structure of the platinum catalyst. The oxygen content (0.1-25%), especially in combination with higher power, is insufficient to obtain a nano-structured catalyst with a large specific surface area and low platinum content. This technical solution based on the technology of metal cathodic sputtering is intended to produce thick catalytic layers (0.1-1 μm) on the inert metal surface (substrate) having an electrical valve-like action. The primary task of the first stage of application is to reduce the contact resistance and apply a dense coat, and that of the second stage is to apply a coat 0.1 to 1 μm thick (100-1000 nm) with increased dispersity. As seen from the above examples, this electrode is used as an electrolytic cell anode to produce gaseous chlorine. These catalytic coats are formed on inert metals (titanium, niobium, tantalum) and the catalytic coating with platinum microstructure allows reducing anode polarization.